1. Field of the Invention
The present invention relates to a photomask material and a method of processing it. More specifically, the present invention relates to a photomask material which has high-definition image reproducibility and gives a photomask having good image strength, heat resistance and durability against light (ultraviolet light) and which is for use to form a circuit pattern, etc., and a method of effectively processing the above photomask material.
2. Related Art Statement
In recent years, with finer patterns of printed wiring boards, the substrate of a photomask for producing printed wiring boards is shifting from a film to a glass. Further, a photomask for producing a liquid crystal display or LSI is required to use a glass as a substrate due to requirements of high accuracy.
As a photomask material using a glass as a substrate, a so-called dry plate having a substrate and a silver halide emulsion layer formed thereon (to be referred to as "emulsion mask" hereinafter) is less expensive and simple. When the silver halide photographic photosensitive layer of an emulsion mask is exposed to light, an exposed silver halide forms a black image by development. An emulsion mask to which silver salt photography is applied is easily highly sensitized for use with a laser light source, and the system thereof is simple. Since, however, black-coloring silver is dispersed in a layer having a relatively large thickness of 2 to 6 .mu.m, the above emulsion mask is poor in reproduction of a fine image due to the thickness of the image even if a photographic processing for increasing a contrast is carried out, so that it is not suitable for an image having a size of 10 .mu.m or less.
As another photomask material using a glass as a substrate, a photomask material for constituting an image of a hard metal thin layer (to be referred to as "hard mask" hereinafter) is available. The light shielding layer of a hard mask is a metal thin layer, and the mainstream thereof is a chromium layer having a thickness of approximately 0.1 .mu.m. The hard mask is therefore excellent in reproduction of a fine image having a size of approximately 1 .mu.m. However, the steps of etching, etc., are complicated, and no photomask can be easily obtained from the hard mask at present, so that the problem is that the photomask is expensive. Moreover, the hard mask is still insufficient in sharpness and stability in the steps of forming a resist layer of a highly sensitive photopolymer and exposing, and it is not feasible for exposing a fine image directly with a laser light source. Further, chromium involves an environmental problem, and it is forced to employ other methods in recent years.
In JP-A-12-10258, the present inventors have proposed a photomask material (to be referred to as "DTR mask" hereinafter) which gives a photomask having a silver image directly formed on a transparent substrate by a diffusion transfer method. A photomask obtained from the DTR mask is free from discoloration and alteration under heat or ultraviolet light in the continuous use thereof as a photomask since it contains no gelatin, and further, the DTR mask gives a photomask having an image constituted of a metal thin layer by relatively simple processing. It can be therefore easily assumed that the DTR mask has a potential for meeting with the formation of finer patterns hereafter. For being equal or superior to existing photomask materials such as an emulsion mask and a hard mask, however, the DTR mask is required to have the capability of reproduction of a higher-definition image, high image strength, high heat resistance, high durability against light (ultraviolet light), and the like. The capability of reproduction of a high-definition image reproduction includes various factors such as the capability of reproducing a shape and size (line width, etc.), a sharpness of edges and a presence or absence of black spots in a non-image area. No prior art has complied thereto, and it has been and is expected to develop novel techniques. Concerning the image strength, the problem is that metal silver constituting an image on a DTR mask has poor image strength as compared with a chromium image of a chromium mask, so that the image is liable to be damaged when the exposure of a photomask obtained from the DTR mask to light is repeated a number of times while being in contact with photosensitive materials.